A microsegregation model in the two-phase region of an ND steel continuous casting billet
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摘要: 通過構建ND鋼連鑄坯凝固兩相區內溶質的微觀偏析模型, 不僅研究了C、S和P元素對固液兩相區內鋼的高溫力學參數以及溶質再分配的影響, 還對P元素偏析比隨冷卻速率(CR) 的變化規律進行了探究.通過分析模型結果表明: 初始C的質量分數在0.075%~0.125%之間時, 隨著初始C含量的增加, P、S元素的偏析加劇, 凝固末端溫度下降幅度變大, 導致脆性溫度區間增大; 增加P和S元素的初始含量, P、S元素的偏析比降低, 但會加劇其在枝晶間殘余液相中的富集, 直接導致零塑性溫度(ZDT) 下降; ND鋼中的Cu含量低于顯著提高裂紋敏感性的臨界含量, 且凝固過程中Cu元素的偏析比較低, 因此在ND鋼凝固過程中Cu元素不能主導裂紋的誘發; 在一定的冷卻速率波動范圍內, P元素的偏析比隨著冷卻速率(CR)的提高略有下降.Abstract: ND steel is a low alloy steel that resists the dew point corrosion of sulfuric acid.To improve the special performance of ND steel, the chemical composition of ND steel not only contains conventional elements but also adds corrosion-resistant elements, such as Cu, Cr, and Ni.During the solidification process, the molten steel will undergo a phase change reaction.Owing to the differences in the distribution coefficients and diffusion coefficients of solute elements in different phases, solute elements will be redistributed in the solid-liquid two-phase region during solidification, which will lead to microsegregation of solute elements.The microsegregation of solute element makes the zero strength temperature and zero plasticity temperature (ZDT) of steel decrease, which makes the temperature range of brittleness expand and deteriorates the mechanical property of high temperature of the continuous casting billet, and finally increases the probability of inducing surface cracks.This paper takes the microsegregation of solute elements as the research background.Herein, a microsegregation model for the solute in the solidified two-phase region of an ND steel continuous casting billet was established.In the model, the effects of elements C, S, and P on high-temperature mechanical parameters and solute redistribution of steel in its solid-liquid two-phase region were studied, and the variation law of the segregation ratio of elemental P with cooling rate (CR) was also explored.According to the analysis of the model results, when the initial C content was between 0.075%and 0.125%, with an increase in the initial C content, segregation of P and S elements intensified, and the temperature drop at the solidification end became larger, leading to the increase in the brittle temperature range.According to the analysis of the model results, increasing the initial content of P and S will decrease the segregation ratio of P and S elements but will increase the enrichment content of P and S elements in the residual liquid phase between dendrites, directly leading to the decline of ZDT.Analysis of the model results shows that the Cu content in ND steel is lower than the critical content that significantly increases the crack sensitivity, and the segregation ratio of Cu element is at a low level during solidification.Therefore, elemental Cu cannot dominate the induced crack in ND steel during solidification.Finally, within a certain range of cooling rate fluctuation, the segregation ratio of P will decrease slightly with increasing CR.
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圖 1 枝晶間液相中P質量分數隨固相率的變化
Figure 1. Change of P mass fraction in the intercrystalline liquid phase with the solid phase ratio
圖 2 ND鋼枝晶間殘余液相凝固點溫度隨固相率的變化
Figure 2. Changes in temperature of the residual liquid phase solidifica-tion point of ND steel dendrite with solid phase ratio
圖 3 凝固過程中枝晶間各元素偏析比變化
Figure 3. Segregation ratio of each element of the dendrites in the solid-ification process
圖 4 C質量分數對溶質偏析比的影響. (a) f=0.75; (b) f=0.99
Figure 4. Effect of C mass fraction on solute segregation ratio: (a) f=0.75; (b) f=0.99
圖 5 C質量分數對零強度溫度(ZST) 和零塑性溫度(ZDT) 的影響
Figure 5. Influence of C mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
圖 6 S質量分數對枝晶間S元素偏析的影響. (a) f=0.75; (b) f=0.99
Figure 6. Influence of S mass fraction on the segregation of S elements in dendrites: (a) f=0.75; (b) f=0.99
圖 7 S質量分數對零強度溫度(ZST) 和零塑性溫度(ZDT) 的影響
Figure 7. Influence of S mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
圖 8 P質量分數對枝晶間P元素偏析的影響. (a) f=0.75; (b) f=0.99
Figure 8. Influence of P mass fraction on the segregation of P elements in dendrites: (a) f=0.75; (b) f=0.99
圖 9 P質量分數對零強度溫度(ZST) 和零塑性溫度(ZDT) 的影響
Figure 9. Influence of P mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
表 1 1873K時各元素活度相互作用系數
Table 1. Activity interaction coefficient of each element at 1873K
eij C Si Mn P S Cu Ni Cr Mn -0.07 - - -0.0035 -0.048 - - - S 0.11 0.063 -0.026 0.029 -0.028 -0.0084 0 -0.011 
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表 2 溶質元素在各相中的凝固參數
Table 2. Solidification parameters of the solute elements in each phase
元素 kiδ/L kiγ/L Diδ/(cm2·s－1) Diγ/(cm2·s－1) mi C 0.19 0.34 5.08×10-5 8.26×10-6 78.0 Si 0.77 0.52 3.70×10-7 1.17×10-8 7.6 Mn 0.76 0.78 1.86×10-7 2.47×10-9 4.9 P 0.23 0.13 4.81×10-7 4.10×10-8 34.4 S 0.05 0.035 2.16×10-6 6.27×10-7 38.0 Cu 0.53 0.88 2.21×10-7 2.63×10-9 5.32 Ni 0.83 0.95 1.36×10-7 1.63×10-10 4.69 Cr 0.95 0.86 2.239×10-7 4.236×10-10 1.04
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表 3 實驗鋼種化學成分(質量分數)
Table 3. Chemical composition of experimental steel?
% C Si Mn P S 0.13 0.35 1.52 0.016 0.002
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表 4 ND鋼化學成分(質量分數)
Table 4. ND steel element composition?
% C Si Mn P S Cu Ni Cr 0.07~0.125 0.20~0.40 0.40~0.60 ＜0.025 ＜0.010 0.25~0.45 0.10~0.20 0.75~1.00
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